A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion on a substrate, e.g., a silicon wafer. Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once. Conventional lithographic apparatus include so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
For measuring displacements in a lithographic apparatus, there is provided a displacement measuring system that is configured to determine a position of the substrate and/or the patterning device. The displacement measuring system may have at least one encoder constructed and arranged to measure a position with respect to a grid structure provided with a repetitive pattern. It may be difficult to manufacture a grid structure at the required size and precision.